It's what we used for passenger in the airline industry over 20 years ago as well. It seemed everything was reduced to a 3 letter code for the airlines. First airport/city codes, then abbreviations within the company, and also for other employee things like DAT (day at a time vacation) and *** (not the offensive term) for without pay (meaning getting off early without the pay). Lots of them.
I found this one on wordorigins.org, and it sums up what I understood ... for example, delays/cancellations of flights are referred to as WX (weather) or MX (mechanical), to expand upon what Ben correctly noted above:
"I suspect PAX is may well come originally from the airline industry. Abbreviations ending in X are quite widely used: WX for weather, CANX for cancelled and so on. I think these may in turn have originated in wireless telegraphy (in morse), where X abbreviations are still used (particularly by ham radio operators), for example TX for transmitter, RX for receiver, DX for distance. (My wife used to work as flight operations secretary at Plymouth airport and still writes CANX on our kitchen calendar to indicate events that have been cancelled.)"
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